PTL 1 describes the following method of effectively using an existing second storage controller when a storage controller is newly introduced to a data center or the like for the purpose of achieving higher performance and larger capacity. A host computer and a first storage controller are coupled via a first communication path. The first storage controller and a second storage controller are coupled via a second communication path. The first storage controller receives a first data I/O request from the host computer via the first communication path. In the case where the first storage controller determines that the first storage controller is not in charge of the first data I/O request, the first storage controller transmits a second data I/O request corresponding to the first data I/O request to the second storage controller via the second communication path. Then, the second storage controller receives the second data I/O request, and performs data I/O processing corresponding to the second data I/O request.